The invention relates to a vacuum sewer arrangement.
In a vacuum sewer arrangement, a toilet bowl is connected to a sewer by means of a sewer valve, and a vacuum is maintained in the sewer. In order to carry out a flushing operation, the sewer valve is opened and waste in the toilet bowl is inducted into the sewer. The flushing operation includes the following phases:
a) the sewer valve opens, PA0 b) the waste in the bowl is pressed into the sewer by the pressure difference between the sewer and the toilet bowl, PA0 c) air enters the sewer behind the waste, and PA0 d) the sewer valve closes.
The waste forms a movable plug in the vacuum sewer, and the pressure difference across the plug propels the plug through the sewer to a waste receiving tank. In a conventional vacuum sewer arrangement, air for transportation of the waste plug enters the sewer by way of the sewer valve. When the sewer valve closes, the plug stops moving almost immediately.
Two main factors determine how long the sewer valve remains open. First, the sewer valve must remain open for long enough to ensure, with a reasonable degree of certainty, that the waste in the toilet bowl has entered the sewer. Although normally the waste is pressed into the sewer almost instantaneously, it is usual to allow about two seconds for the waste to enter the sewer. Second, it is important for proper operation of a vacuum sewer arrangement that the chance of a waste plug merging with a preceding or succeeding plug in the sewer by very small, and therefore it must travel a substantial distance along the sewer, for example at least 10 m, before stopping. In vacuum sewer arrangements that are currently in use, the waste plug typically travels at a maximum speed of about 5-10 m/s. Therefore, in a typical vacuum sewer arrangement, the sewer valve remains open for about four seconds on each flush.
The waste will enter the sewer immediately the sewer valve opens, followed by a large quantity of air. This causes considerable pressure variations in the toilet bowl, in particular during the opening and closing phase of the sewer valve when material (waste or air) is inducted through a relatively small opening. Due to this, a high noise level is generated. The noise level is dependent on the pressure difference between the toilet bowl and the sewer--the greater the difference, the greater the noise.
Krishnakumar et al, U.S. Pat. No. 4,791,688 issued Dec. 20, 1988, discloses a vacuum sewer arrangement comprising multiple toilet bowls connected through respective sewer valves and sewers to a transfer manifold. When a flush control for one of the toilet bowls is actuated, a macerator pump starts recirculating waste from a waste receiving tank through a jet pump whose suction inlet is connected to the transfer manifold. Within about 3 seconds, vacuum is built up in the manifold. A controller device then opens a toilet flush valve for supplying rinse water to the toilet bowl, and subsequently opens the toilet bowl's sewer valve. Waste is drawn from the toilet bowl into the sewer and is transported into the manifold. After a few seconds of evacuation, the controller closes the sewer valve, and about 1 second later closes the flush valve, discontinuing supply of rinse water. A solenoid-operated air inlet valve is then opened, allowing air to flow directly into the manifold. The air inlet valve remains open for several minutes, and the controller then shuts off the macerator pump and closes the air inlet valve.
The air supplied by the air inlet valve of Krishnakumar et al transports the waste from the transfer manifold into the holding tank, but does not aid in transport of sewage from the toilet bowl to the transfer manifold. Since the air inlet valve opens at an unspecified time after the sewer valve closes, operation of the air inlet valve can have no effect on the process of emptying the toilet bowl, and hence has no effect on the noise level. Further, since the distance of the solenoid valve from the sewer valve is different for the different toilet bowls, it is evident that there is not a timed relationship, uniform for all the toilet bowls, between the time that the sewer valve opens and the time that the waste is placed under the influence of air that has entered through the air inlet valve.
The description of the system disclosed by Krishnakumar et al does not refer either to the waste being transported in the form of movable plugs or to a reduction of the noise level.